It is known in the field of optical speed sensors that optical fibers may be used to measure the rotational speed of gear teeth. Fiber optic sensing techniques, as is known, offer an advantage over conventional electrical sensors, e.g., magnetic pick-ups, by providing immunity to electromagnetic interference, reduced weight, and a wide signal bandwidth.
Prior art fiber optic techniques include intensity sensors, polarization modulation sensors, and Doppler sensing techniques.
Intensity type sensors provide a light beam on one side of a gear tooth and an optical detector on the opposite side of the gear tooth. When the tooth passes by the sensor, it blocks the light beam and causes the detection signal to drops out, thereby indicating the presence of gear tooth. This is also known as beam chopping. In that case, any contamination e.g., dirt, grease or particulate matter that collects on the beam source or detector may cause the system to become inoperable.
Polarization modulation sensors provide source light which exits an optical fiber and is incident on a collimating lens (e.g., a GRIN lens) which collimates the beam. The collimated beam passes through a polarizer. The polarized light passes through a clear rare-earth material, which is located near a permanent magnet or other source of magnetic field, and which alters the polarization of the polarized light in response to a change in magnetic fields therein. The light exits the rare-earth material and is reflected off of a mirror and back through the rare earth material, the polarizer, and the lens and onto a return fiber. As the gear tooth passes, the increased magnetic field strength through the rare-earth material alters the polarization, thereby reducing the power of the return light that passes through the polarizer.
With polarization sensors, however, because light exits the fiber (to be incident on bulk optics), precise alignment of the optical fibers with the optical elements is required. Also, this method is expensive and complex because uses polished rare earth garnets, coated GRIN rod lenses and polished fiber faces.
Doppler sensors provide source light incident partially tangentially on a rotating part and detect light back-scattered from the part. The frequency of the detected light is shifted from the source light in a known way that indicates the angular velocity of the part. To detect this frequency shift at a frequency detectable with electronics, FM modulation techniques are used at both the optical source, e.g., using an acousto-optic modulator or an integrated optic phase modulator, and in the detection circuitry, e.g., using standard frequency demodulation techniques.
However, to provide the needed feedback light the part must be coated with retro-reflective paint. Also, the speed calculation is dependent on the tangential angle with which the source light is incident on the target, thereby making use of a hand-held or tripod device prohibitive. Also, this technique requires complex, high-cost signal processing hardware and/or software. Further acousto-optic modulators can be bulky and have high power requirements, e.g., two watts, while integrated optic phase modulators are very expensive. Still further, as with the aforementioned intensity devices, the system is sensitive to any contamination which may collect on the shaft (thereby reducing adequate reflected light) or on the beam source and may cause the system to become inoperable.
Thus, it would be desirable to provide an optical speed sensor which overcomes the drawbacks of the prior art.